In recent years, there has been an increasing demand for providing polymer compositions for the production of tires for the automobile industry which allow an improvement of the tire's performance. Particularly, environmental aspects, i.e. reduction of the fuel consumption and/or carbon dioxide emission by reducing the tire's rolling resistance, and safety aspects, i.e. improving the handling stability by increasing the tire's grip performance and abrasion resistance, became more important.
A typical rubber formulation for the manufacturing of a tire includes an aromatic vinyl-conjugated diene copolymer of high molecular weight, such as a styrene butadiene rubber (SBR), which after compounding with several additives, such as a silica filler and a vulcanizing agent, and vulcanization (crosslinking) leads to a tire product having a lower rolling resistance due to the high molecular weight of the SBR component used. Even though providing a tire having an enhanced (lower) rolling resistance is beneficial, the use of an aromatic vinyl-conjugated diene copolymer having a high molecular weight, and for this reason being associated with a high viscosity, results in less beneficial processability in the course of the further processing steps, i.e. compounding and vulcanization (crosslinking) of the corresponding polymer formulations.
Therefore, a high molecular weight aromatic vinyl-conjugated diene copolymer is typically extended with an extender oil of low molecular weight for reducing the copolymer's viscosity and guaranteeing a good mixing, incorporation and distribution of the copolymer in the subsequent compounding and/or crosslinking (vulcanization) steps of the rubber formulation. Typical extender oils (or softeners) are mineral oils and treated mineral oils, such as for example DAE (Distillate Aromatic Extract), TDAE (Treated Distillate Aromatic Extract), MES (Mild Extraction Solvate), RAE (Residual Aromatic Extract) and naphthenic oils. However, there are drawbacks associated with the extension of an aromatic vinyl-conjugated diene copolymer with a standard low molecular weight extender oil:
Since the molecular weight of the common extender oil is relatively low, e.g. about 450 g/mol (converted to polystyrene equivalents) for TDAE, the enhanced processability of the high molecular weight aromatic vinyl-conjugated diene copolymer during the subsequent compounding and vulcanization steps, as mentioned above, is accompanied by a significant increase of the amount of “volatile organic compounds” (herein abbreviated as VOC) in the corresponding final tire product. These VOC emissions become particularly relevant during use of the tire at elevated temperatures.
Another drawback is that the glass temperature and compatibility of these extender oils is more or less fixed and not variable. Thus, when formulating an aromatic vinyl-conjugated diene copolymer, compatibility and subsequent blooming of the extender oil on the vulcanizate surface during storage are relevant factors to be considered. In addition, performance and application field of the vulcanizate (i.e. the crosslinked rubber formulation after vulcanization), which typically depends on the glass transition temperature, may only be adjusted by variation of the specific composition of the aromatic vinyl-conjugated diene copolymer. However, an increase of the glass transition temperature of the aromatic vinyl-conjugated diene copolymer results in an improved grip performance and a higher rolling resistance, thereby increasing fuel consumption and carbon dioxide emission. On the other hand, reduction of the glass transition temperature of the aromatic vinyl-conjugated diene copolymer results in an improved rolling resistance and a lower fuel consumption of the resulting tire, but unfortunately also leads to a reduced wet grip performance.
Another drawback of using a common low molecular weight extender oil is that most of these extender oils are intensely colored, ranging from yellow to deep brown. Accordingly, an intense cleaning of the production plant has to be applied prior to changing polymer grades, particularly if a non-oil extended polymer grade is planned for the next production cycle. Such cleaning procedures are, however, time and cost consuming.
Moreover, the extension of a high molecular weight aromatic vinyl-conjugated diene copolymer with a common extender oil having a low molecular weight results in a less beneficial reduction of the mechanical properties, in particular the modulus, hardness versus handling performance (E′ @ 60° C.), increased abrasion loss and decreased rebound resilience at higher temperatures which correspond to a reduced rolling resistance.
Thus, there is a demand for providing alternative extender components as suitable replacement of or in addition to the common extender oils as applied by the prior art, thereby allowing the provision of cross-linked (vulcanized) polymer formulations, which are characterized by having an acceptable or improved processability, while the content of VOC emissions is reduced and a better balance of the dynamic properties of the crosslinked polymer formulations and the reinforcement is provided, such as a low hysteresis loss, as represented by a low heat build-up, high rebound at higher temperatures and a low tan δ at 60° C., and a higher reinforcement, as represented by higher moduli especially at 300% elongation and high abrasion resistance in the resulting tire products. These needs have been met by the following invention.